this post was submitted on 16 Jan 2024
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I looked into this a bit more and here is the summary: This is meant to show off a candidate kernel feature that allows for running different schedulers in userland.
Task scheduling has become much more complex as CPUs have grown in size and have had new developments in architecture, so the need to develop more complex and robust schedulers is steadily rising.
The kernel feature is meant to lower the barrier of entry for anyone who wants to try getting into schedulers, as well as enable quicker development iteration, by removing the need to completely recompile the linux kernel every time you want to test your code.
Read more at the main project's github: https://github.com/sched-ext/scx
Not quite running in userspace. To the best of my own understanding:
The new kernel feature is to allow writing schedulers in eBPF, a "language" the kernel runs in kernelspace that is heavily restricted.
For example, all eBPF programs must complete in bounded time, and the kernel's static checker must be able to verify that before the program can even begin executing. eBPF is a rare language that is not touring complete.
"For scx_simple, suspending the scheduler process doesn't affect scheduling behavior because all that the userspace component does is print statistics. This doesn't hold for all schedulers."
So, it may be that eBPF also makes it easier to write a truly userspace scheduler, but that's not the primary purpose, and it's not what is being done with scx_simple.
https://lwn.net/Articles/909095/ for more about (e)BPF.
Thank you for the correction! Reading up on eBPF is fascinating.
Additional resource that adds to your secondary point that this is more than just allowing schedulers to be run in userland: https://github.com/sched-ext/scx/blob/main/scheds/c/README.md
Is eBPF dependently typed?